Nematoda

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Roundworm, also nematode, common name for any of a phylum of unsegmented terrestrial, freshwater, or marine worms. Roundworms are almost worldwide in distribution and are abundant in the surface layers of soils. Many of them are economically and medically harmful, living as parasites in plants and animals, including humans. Roundworm infections are common and frequently go unnoticed, but several species cause serious diseases.

Roundworms are cylindrical, tapering animals with simple bodies consisting of an interior gut and a muscular outer wall, separated by a fluid-filled cavity called a pseudocoel (see Animal: Coelom). The outer wall secretes an elastic cuticle that is molted four times during the animal's lifetime. Species range in size from microscopic to about 10 cm (about 4 in) long. Most species have separate sexes, but a few are hermaphroditic; fertilization is internal. The young roundworms, which resemble the adults, develop without metamorphosis.

Although numerous roundworms are free-living, the parasitic forms are of greatest economic interest. One important group, the ascaroid nematodes, includes the threadworms and the common worm of puppies. Another contains the eelworms, which produce root knot of cotton, and forms that produce earcockle of wheat. Other, medically significant forms of roundworm include the various genera known as hookworm; the filaria, which cause elephantiasis; the trichina worm, the cause of trichinosis; and the whipworm, which infests the human intestine.

Scientific classification: Roundworms make up the phylum Nematoda. Threadworms belong to the genus Ascaris. The common worm of puppies is classified as Toxocara canis. Eelworms belong to the genus Heterodera, the forms that produce earcockle of wheat belong to the genus Tylenchus, and the trichina worm belongs to the genus Trichinella. The human whipworm is classified as Trichuris trichiura.

Types of Invertebrates

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Kinorhyncha or Echinodera -> Tiny worms with spiny bodies. An outer protective cuticle is segmented and articulated. Found in the muddy bottoms of coastal waters, they feed on microorganisms and organic particles by means of a sucking pharynx. Reproduction is sexual. Examples: Echinoderes, Condyloderes

Nematoda -> Commonly known as roundworms, these animals are one of the most diverse and geographically widespread invertebrate phyla. Free-living roundworms inhabit freshwater and marine habitats, as well as soil. Parasitic roundworms prey on both plants and animals, causing widespread agricultural damage and disease. Roundworms have long, cylindrical bodies with a mouth surrounded by lips and sensory papillae or bristles. Fluid in the body cavity distributes nutrients and oxygen—roundworms do not have special respiratory or circulatory systems. Roundworms prey on other invertebrates as well as diatoms, algae, and fungi. They reproduce sexually and larvae undergo at least four molts before reaching their adult size and shape. – Examples: Ascarids, vinegar eels, cyst nematodes, heartworms, hookworms

Nematomorpha -> Long, threadlike worms found in soil or freshwater, commonly known as horsehair worms. They have no distinct head. Larvae are parasitic on terrestrial arthropods, usually insects. Adults do not feed but depend entirely on nutrients obtained during the parasitic larval stage. Reproduction is sexual. – Examples: Nectoneme, gordian worms

Acanthocephala -> Worms characterized by the presence of retractable spiny hooks that attach to the intestinal walls of aquatic and terrestrial vertebrates. Lacking a digestive tract, these worms absorb nutrients through their body wall. Reproduction is sexual and the life cycle includes larval forms that are parasites of crustaceans or insects. – Examples: Spiny-headed worms

Gnathostomulida -> Elongated, ciliated worms that live in marine sand. These animals have a mouth structure with a combed plate and toothed jaws. Bacteria and fungi are scraped into the mouth by the comb and passed into the gut by snapping movements of the jaws. Hermaphroditic, these animals reproduce sexually. – Examples: Jaw worms

Mollusca -> Diverse animals found in water and on land. Most mollusks have a hard shell that protects a soft body, although in some mollusks the hard shell is missing or hardly visible. A feeding organ called a radula contains rows of teeth used to scrape food into the mouth. Enzymes in salivary glands partially digest food before it reaches the intestines. Reproduction is sexual and some mollusks have a larval form. – Examples: Chitons, oysters, snails, clams, squid

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